732 A MANUAL OF VETERINARY PHYSIOLOGY 



contribute between them one quarter, or each of them one-sixteenth, 

 and so on, the sum of the series, |+ 1+ ^+ ,V+ > being equal 

 to i, as it should be. The bias towards particular diseases, or ten- 

 dencies to such diseases, by any particular ancestors in any given 

 pedigree are eliminated by a law which deals only with average 

 contributions, and the various prepotencies of sex in respect to 

 different qualities, such as the familiar sex-limited diseases of 

 pseudo-hypertrophic muscular paralysis, haemophilia, and colour- 

 blindness, are also presumably eliminated/ The essential feature 

 to be borne in mind in connection with Galton's law is that it applies 

 to masses rather than to the individual. It is an average result, the 

 immediate parents in all cases being more largely represented than 

 any of the other progenitors. For the improvement of a breed it 

 is essential, according to Galtonism, that the laws of artificial selection 

 should prevail, which, acting over a sufficient length of time, would 

 blend in desirable proportions the special qualities required in the 

 mass, though they might be absent in the individual. 



Weismann's Germ-Plasm Theory of heredity broke away from the 

 Darwinian view of the transmission of modifications from parent 

 to offspring. He held that the use or disuse of any part, which under 

 the Lamarckian doctrine would be transmitted to the next genera- 

 tion, is not so transmitted. In other words, there was no proof of 

 acquired or somatic characters being heritable, and that the cases 

 which had been brought forward to support this view could reason- 

 ably be explained by selection. Selection, he urged, was the prime 

 factor in producing racial change, while environment had little, if 

 any, influence. The germ plasm, he pointed out, was continued 

 from generation to generation, but is subject to inborn transforma- 

 tion, and the environment selects the fittest or most suitable plasm 

 through its finished product, the soma, the modifications so selected 

 being transmitted to the next generation. 



The Mutation Theory of de Vries was in sharp contrast to the 

 fundamental principle of Darwinism, which laid down that changes 

 were extremely slow and gradual. De Vries showed that in the case 

 of certain plants variation arose suddenly. In consequence of this 

 discovery the view that evolution was the result of minute differ- 

 ences acting through long periods was no longer regarded as neces- 

 sary. Bateson had previously shown that the difference between 

 species was not a gradual but a sharp one, and that intermediates 

 did not exist. The continuity of the germ-plasm, on which Weis- 

 mann took his stand, was thus not only called into question, but its 

 exact opposite — *.*., its discontinuity — demonstrated. 



Mendelism. — The latest view of heredity is that put forward by 

 Mendel many years ago, though it remained unrecognised until 

 recently. It is difficult to define it in precise terms until examined, 

 but it aims at applying physiological laws of inheritance to the 

 individual rather than to the mass. It does not at present, and 

 may never be able to, deal with all the characteristics of individuals. 

 In the case of animals the application of Mendelian principles is as 

 yet somewhat limited. Mendelian inheritance is known to exist in 

 the matter of coat colour and length of fur in rabbits, and of the 

 fleece in sheep. It applies to the coat of various breeds of dogs — e.g., 

 basset hounds and terriers. It also applies to eye colour. In the case 

 of cattle and sheep it applies to coat colour and horns, and in dairy 

 stock, it is said, to the quantity and quality of milk.* In poultry, to 



* Professor J. Wilson, M.A., Proceedings of the Royal Dublin Society, 

 1910-11. 



